include ../_util-fns

:marked
  Our app is growing.
  Use cases are flowing in for reusing components, passing data to components, and creating more reusable assets. Let's separate the heroes list from the hero details and make the details component reusable.

  Run the <live-example></live-example> for this part.

.l-main-section
:marked
  ## Where We Left Off
  Before we continue with our Tour of Heroes, let’s verify we have the following structure. If not, we’ll need to go back and follow the previous chapters.

.filetree
  .file angular_tour_of_heroes
  .children
    .file lib
    .children
      .file app_component.dart
    .file web
    .children
      .file index.html
      .file main.dart
      .file styles.css
    .file pubspec.yaml
:marked
  ### Keep the app compiling and running
  We want to start the Dart compiler, have it watch for changes, and start our server. We'll do this by typing

code-example(language="bash").
  pub serve

:marked
  This will keep the application running while we continue to build the Tour of Heroes.

  ## Making a Hero Detail Component
  Our heroes list and our hero details are in the same component in the same file.
  They're small now but each could grow.
  We are sure to receive new requirements for one and not the other.
  Yet every change puts both components at risk and doubles the testing burden without benefit.
  If we had to reuse the hero details elsewhere in our app,
  the heroes list would tag along for the ride.

  Our current component violates the
  [Single Responsibility Principle](https://blog.8thlight.com/uncle-bob/2014/05/08/SingleReponsibilityPrinciple.html).
  It's only a tutorial but we can still do things right &mdash;
  especially if doing them right is easy and we learn how to build Angular apps in the process.

  Let’s break the hero details out into its own component.

  ### Separating the Hero Detail Component
  Add a new file named `hero_detail_component.dart` to the `lib` folder and create `HeroDetailComponent` as follows.

+makeExample('toh-3/dart/lib/hero_detail_component.dart', 'v1', 'lib/hero_detail_component.dart (initial version)')(format=".")
.l-sub-section
  :marked
    ### Naming conventions
    We like to identify at a glance which classes are components and which files contain components.

    Notice that  we have an `AppComponent` in a file named `app_component.dart` and our new
    `HeroDetailComponent` is in a file named `hero_detail_component.dart`.

    All of our component names end in "Component".  All of our component file names end in "_component".

    We spell our filenames in lower **underscore case**
    (AKA **[snake_case](../guide/glossary.html#snake_case)**) so we don't worry about
    case sensitivity on the server or in source control.

    <!-- TODO
    .l-sub-section
      :marked
        Learn more about naming conventions in the chapter [Naming Conventions]
    :marked
    -->
:marked
  We begin by importing the Angular `core.dart` file,
  so that we can use common types like `@Component` when we create
  our component.

  We create metadata with the `@Component` annotation where we
  specify the selector name that identifies this component's element.

  When we finish here, we'll import it into `AppComponent` and create a corresponding `<my-hero-detail>`  element.
:marked
  #### Hero Detail Template
  At the moment, the *Heroes* and *Hero Detail* views are combined in one template in `AppComponent`.
  Let’s **cut** the *Hero Detail* content from `AppComponent` and **paste** it into the new template property of  `HeroDetailComponent`.

  We previously bound to the `selectedHero.name` property of the `AppComponent`.
  Our `HeroDetailComponent` will have a `hero` property, not a `selectedHero` property.
  So we replace `selectedHero` with `hero` everywhere in our new template. That's our only change.
  The result looks like this:

+makeExample('toh-3/dart/lib/hero_detail_component.dart', 'template', 'lib/hero_detail_component.dart (template)')(format=".")

:marked
  Now our hero detail layout exists only in the `HeroDetailComponent`.

  #### Add the *hero* property
  Let’s add that `hero` property we were talking about to the component class.
+makeExample('toh-3/dart/lib/hero_detail_component.dart', 'hero')
:marked
  Uh oh. We declared the `hero` property as type `Hero` but our `Hero` class is over in the `app_component.dart` file.
  We have two components, each in their own file, that need to reference the `Hero` class.

  We solve the problem by relocating the `Hero` class from `app_component.dart` to its own `hero.dart` file.

+makeExample('toh-3/dart/lib/hero.dart', '', 'lib/hero.dart')(format=".")

:marked
  Add the following import statement near the top of **both `app_component.dart` and `hero_detail_component.dart`**.

+makeExample('toh-3/dart/lib/hero_detail_component.dart', 'hero-import')

:marked
  #### The *hero* property is an ***input***

  The `HeroDetailComponent` must be told what hero to display. Who will tell it? The parent `AppComponent`!

  The `AppComponent` knows which hero to show: the hero that the user selected from the list.
  The user's selection is in its `selectedHero` property.

  We will soon update the `AppComponent` template so that it binds its `selectedHero` property
  to the `hero` property of our `HeroDetailComponent`. The binding *might* look like this:
code-example(format=".").
  &lt;my-hero-detail [hero]="selectedHero">&lt;/my-hero-detail>
:marked
  Notice that the `hero` property is the ***target*** of a property binding &mdash; it's in square brackets to the left of the (=).

  Angular insists that we declare a ***target*** property to be an ***input*** property.
  If we don't, Angular rejects the binding and throws an error.
.l-sub-section
  :marked
    We explain input properties in more detail [here](../guide/attribute-directives.html#why-input)
    where we also explain why *target* properties require this special treatment and
    *source* properties do not.
:marked
  There are a couple of ways we can declare that `hero` is an *input*.
  We'll do it the way we *prefer*, by annotating the `hero` property with `@Input()`.
+makeExample('toh-3/dart/lib/hero_detail_component.dart', 'inputs')(format=".")

.l-sub-section
  :marked
    Learn more about `@Input()` in the
    [Attribute Directives](../guide/attribute-directives.html#input) chapter.

.l-main-section
:marked
  ## Refresh the AppComponent
  We return to the `AppComponent` and teach it to use the `HeroDetailComponent`.

  We begin by importing the `HeroDetailComponent` so we can refer to it.

+makeExample('toh-3/dart/lib/app_component.dart', 'hero-detail-import')(format=".")

:marked
  Find the location in the template where we removed the *Hero Detail* content
  and add an element tag that represents the `HeroDetailComponent`.
code-example(format=".").
  &lt;my-hero-detail>&lt;/my-hero-detail>
.l-sub-section
  :marked
    *my-hero-detail* is the name we set as the  `selector` in the `HeroDetailComponent` metadata.
:marked
 The two components won't coordinate until we bind the `selectedHero` property of the `AppComponent`
 to the `HeroDetailComponent` element's `hero` property  like this:
code-example(format=".").
  &lt;my-hero-detail [hero]="selectedHero">&lt;/my-hero-detail>
:marked
  The `AppComponent`’s template should now look like this

+makeExample('toh-3/dart/lib/app_component.dart', 'hero-detail-template', 'app_component.dart (template)')(format=".")
:marked
  Thanks to the binding, the `HeroDetailComponent` should receive the hero from the `AppComponent` and display that hero's detail beneath the list.
  The detail should update every time the user picks a new hero.

  It's not happening yet!

  We click among the heroes. No details. We look for an error in the console of the browser development tools. No error.

  It is as if Angular were ignoring the new tag. That's because *it is ignoring the new tag*.

  ### The *directives* list
  A browser ignores HTML tags and attributes that it doesn't recognize. So does Angular.

  We've imported `HeroDetailComponent`, we've used it in the template, but we haven't told Angular about it.

  We tell Angular about it by listing it in the metadata `directives` list. Let's add that list property to the bottom of the
  `@Component` configuration object, immediately after the `template` and `styles` properties.
+makeExample('toh-3/dart/lib/app_component.dart', 'directives')(format=".")


:marked
  ### It works!
  When we view our app in the browser we see the list of heroes.
  When we select a hero we can see the selected hero’s details.

  What's fundamentally new is that we can use this `HeroDetailComponent`
  to show hero details anywhere in the app.

  We’ve created our first reusable component!

  ### Reviewing the App Structure
  Let’s verify that we have the following structure after all of our good refactoring in this chapter:

.filetree
  .file angular_tour_of_heroes
  .children
    .file lib
    .children
      .file app_component.dart
      .file hero.dart
      .file hero_detail_component.dart
    .file web
    .children
      .file index.html
      .file main.dart
      .file styles.css
    .file pubspec.yaml
:marked
  Here are the code files we discussed in this chapter.

+makeTabs(`
  toh-3/dart/lib/hero_detail_component.dart,
  toh-3/dart/lib/app_component.dart,
  toh-3/dart/lib/hero.dart
  `,'',`
  lib/hero_detail_component.dart,
  lib/app_component.dart,
  lib/hero.dart
  `)

.l-main-section
:marked
  ## The Road We’ve Travelled
  Let’s take stock of what we’ve built.

  * We created a reusable component
  * We learned how to make a component accept input
  * We learned to bind a parent component to a child component.
  * We learned to declare the application directives we need in a `directives` list.

  Run the <live-example></live-example> for this part.

.l-main-section
:marked
  ## The Road Ahead
  Our Tour of Heroes has become more reusable with shared components.

  We're still getting our (mock) data within the `AppComponent`.
  That's not sustainable.
  We should refactor data access to a separate service
  and share it among the components that need data.

  We’ll learn to create services in the [next tutorial](toh-pt4.html) chapter.
